Method for producing gadolinium molybdate single crystals having high transparency

ABSTRACT

A method for producing gadolinium molybdate single crystals is particularly characterized by its cooling step wherein when passing through a temperature zone of at least 900*C. to 700*C., the molybdate material is cooled at a cooling rate of at least 200*C./hr. According to this method there is obtained gadolinium molybdate single crystals with excellent transparency that is as high as 70 to 82% in light permeability at 6,328 A wavelength. The obtained single crystals are of high utility value and can be most advantageously used for various types of optical materials.

United States Patent [191 Akiyama et al.

[ METHOD FOR PRODUCING GADOLINIUM MOLYBDATE SINGLE CRYSTALS HAVING HIGHTRANSPARENCY [75] Inventors: Seikichi Akiyama, Kokubunji,

Tokyo; Hirotsugu Kozuka, Tokyo, both of Japan [73] Assignee: Hitachi,Ltd., Japan [22] Filed: July 28, 1972 [21] Appl. No.: 276,166

[52] US. Cl 423/263; 23/301 SP; 23/305; 423/593 [51] Int. Cl C01g 39/00;C01g 57/00 [58] Field of Search 423/263, 593; 23/301 SP, 23/304, 305

[56] References Cited UNITED STATES PATENTS 3,437,432 4/1969 Borchardt423/263 [451 July 8,1975

Ballman et al. 23/301 SP Aizu et al. 423/593 X Primary ExaminerHerbertT. Carter Attorney, Agent, or FirmCraig & Antonelli [5 7 ABSTRACT 4Claims, 2 Drawing Figures OOLING RATE (-xlOOC/h) METHOD FOR PRODUCINGGADOLINIUM MOLYBDATE SINGLE CRYSTALS HAVING HIGH TRANSPARENCY BACKGROUNDOF THE INVENTION This invention relates to a method for producinggadolinium molybdate (Ga (MoO single crystals having high transparency.

Gadolinium molybdate single crystals are generally ferroelectric andferroelastic and also lightpenetratable. Consequently, these crystalsprovide very excellent characteristics when used as optical material.

Although these single crystals have many excellent physical propertiesfor use as optical materials as noted above, one problem inmanufacturing such crystals is that it is extremely difficult to obtainhigh quality crystals. This problem is hereinafter discussed in greaterdetail.

It is well known that in order to obtain gadolinium molybdate crystalsof good quality by using the technique of growing single crystals fromthe melt of high temperature according to a pull method (which isgenerally known as Czochralski method), it is usually necessary 3, thespeed at which the single crystal is pulled up onto the melt surface andto also lower the cooling rate. The former step (to reduce the pull-upspeed) is envisaged to minimize the crystal imperfections and separationof impurities, while the latter step (to reduce the cooling rate) isdesigned to preclude occurrence of cracks in the crystals due to heatdistortion and internal stress strain. Usually, these single crystalsare produced in the following way: first, gadolinium oxide andmolybdenum oxide are mixed in the form of powder at a mol ratio of l 3then the mixture is fired at a temperature of around 1,000C., the firedproduct being then melted and a seed crystal is immersed in the melt androtated to raise up a single crystal according to the pull method. Afterthe crystal has grown to a certain extent, it is pulled up from the meltand immediately subjected to slow cooling.

In these single crystals, a sharp crystal deformation takes place ataround the Curie temperature (i.e. 160C.) to give rise to a large strainstress. Consequently, cracks are very liable to occur during the coolingstep in the course of crystal growth, and in some cases, the entirecrystal may be even broken into pieces. It is therefore important forobtaining good quality crystals to reduce the cooling speed as much aspractical. It is also necessary to extremely lower the crystal growingspeed because the crystals are of a polymolecular structure where twodifferent oxides, i.e. gadolinium oxide and molybdenum oxide, arecompounded and also because the mixing rate of these two oxides must bestoichiometrically satisfied. If the single crystal growing is conductedwith due consideration given to the above-said properties of gadoliniummolybdate crystals, there can be obtained single crystals which are freeof cracks and have a stoichiometrical mixing rate. However, these singlecrystals are mostly unsatisfactory in transparency, with their lightpenetrability or permeability being usually within the range of 30 to50%. Such poor penetrability causes the scattering of light chiefly atthe fine granular white points lying sporadically in the crystalstructure.

SUMMARY or THE INVENTION An object of the present invention is to obtaingadolinium molybdate single crystals having high penetrability.According to the present invention, there is provided an improved methodfor producing gadolinium molybdate single crystals having as high as to82% light penetrability. This method is particularlycharacterized by itscooling step in which (as hereinafter described in greater detail) thework mixture is cooled at a specific cooling rate while passing througha specific temperature zone. The single crystal producing methodaccording to the present invention comprises essentially: the step ofheating and melting a polycrystal of gadolinium molybdate (Gd (MoO orheating an admixture of molybdenum oxide and gadolinium oxide blended tomeet a stoichiometrical value for forming said gadolinium molybdate, ata temperature higher than the melting point of said admixture to form amelt; the steps of immersing a seed crystal in said melt, and growing asingle crystal of gadolinium molybdate according to a pull method(Czochralski method); and the step of cooling said single crystal, saidcooling being carried out at a rate of higher than 200C./hr whilepassing through the temperature zone of at least 900C. to 700C. duringsaid cooling step.

Thus, it will be understood that the present invention is based on thefinding of the inventors that the cooling rate in the cooling step afterthe growth of crystal, particularly the cooling rate in the temperaturezone of from 900C. to 700C, exerts a decisive influence to lightpenetrability of the final crystal product.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 diagrammatically illustratesone embodiment of the present invention, showing a light penetrabilitycurve of the single crystal as observed when the cooling rate in thecooling step after pulling up the single crystal is varied; and

FIG. 2 is a characteristic curve showing variation of the lightpenetrability as a single crystal of 35% penetrability is subjectedtoheat treatment at various temperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will be furtherunderstood from the following typical example.

EXAMPLE Gadolinium oxide and molybdenum oxide, both in the form ofpowder, were mixed at the mol ratio of l 3 and well pulverized and mixedup. The powder mixture was then shaped into a small flat plate-like massand fired in the air at 700C. for 4 hours. The fired product was againpulverized into powder, which was then well mixed and again shaped intoa small flat plate-like mass, the formed mass being fired in the air atl000C. for about 4 hours. The fired product thus obtained was thenpacked in a crucible and melted by heating at l,l70C. in the air. Then,a seed crystal was fixed to a platinum support and the (001) face ofsaid crystal was immersed in said melt, and then said support wasgradually pulled up at a speed of about 8 mm/hr while rotating it at aspeed of about r.p.m., whereby a columnar gadolinium molybdate singlecrystal measuring 15 mm in diameter and 50 mm in length was grown. Theobtained c'r'y 'al was t off froiiilthe melt and the crystal temperaturewas maintained at the range of l,020 to 970C. Then, the crystal wascooled from this range of temperature down to a temperature close to theCurie point (160C.) at various cooling rates shown on the abscissa ofthe graph shown in FIG. 1. Light penetrability of the thus obtainedspecimens was measured to obtain a result expressed by the coolingrate-penetrability curve in FIG. 1. Measurement of penetrability wasmade by using a He-Ne laser (output 15 mW, wavelength 6,328 A) as lightsource.

As apparent from the figure, light penetrability is greatly influencedby the cooling rate. Usually, penetrability or permeability of over 60to 70% level is required for an optical material to be servable inpractical use. According to the present invention, as apparent from thecurve of FIG. 1, it is possible to obtain an excellent crystal havingpenetrability of over 70% by se lecting a cooling rate of higher than200C./hr. If a cooling rate of higher than 450C./hr is used in thepresent invention, there can be obtained a crystal having splendidpenetrability of as high as 82% which is close to the theoreticalpenetrability.

As obvious from the cooling rate penetrability curve heretoforedescribed, the light penetrability of the crystals obtained at a coolingrate of higher than 450C./hr is substantially equal to the theoreticalpenetrability (82%). As the results of experiments, it has been foundpossible to obtain a crystal having very excellent light penetrability,with no trace of cracks being created even at an extraordinarily highcooling rate of from 3,000 to 6,000C./hr (50 to lC./mm). However, merelyincreasing the cooling rate above certain limits ends up with badeconomy, so the most practical range of cooling rate is, in most cases,within the range of from 200C. to l,500C./hr.

' As is also evident from the curve shown in FIG. 1, a cooling rate onthe order of l50C./hr or higher is sufficient to obtain a crystal withlight penetrability of 60% or higher.

In the foregoing example, air was employed as the crystal growingatmosphere, but in the present invention, any other oxidizing atmospherecontaining oxygen can be used. Also, the rotational speed of the seedcrystal is not limited to 120 r.p.m. as in the above example, but can besuitably selected from the range of 50 to 180 r.p.m. Further, thecrystal pulling-up speed is not restricted to 8 mm/hr but is selectablefrom the range of l to 5.0 mm/hr. Thus, it will be understood that, inthe present invention, the ordinary conditions for producing a crystalby the pulling method can be used until the point where the crystalgrowth is completed and that the characteristic feature of the inventionresides in the cooling step which is practiced after completion of thecrystal growth.

FIG. 2 shows a heating temperature-penetrability curve which furthershows how light penetrability (i.e. penetrating power) varies when agadolinium molybdate single crystal of 35% light penetrability at normaltemperature (obtained by a known method) is heated gradually to highertemperatures. It is learned from this curve that the temperature zone inwhich the crystal is apt to become non-transparent is between 700C. and

900C. This means that, in the cooling step in the production of singlecrystals, it is essential to minimize the period in which the crystal isleft exposed to the temperature range of from 900C. to 700C. For thisreason, it is required in the present invention to perform the coolingat a cooling speed of over 200C./hr in the temperature zone of 900C. to700C.

As will be appreciated from the foregoing explanation, it is possiblewith the present invention to obtain excellent gadolinium molybdatesingle crystals having light penetrability which is higher than at leastThus, the present invention realizes an amazing enhancement of utilityvalue of said crystals for use as optical materials and greatlycontributes to the benefit of the industries concerned.

While the novel principles of the invention have been described, it willbe understood that various omissions, modifications and changes in theseprinciples maybe made by one skilled in the art without departing fromthe spirit and scope of the invention.

What is claimed is:

1. In a method for producing gadolinium molybdate single crystalswherein a polycrystal of gadolinium molybdate or a mixture of molybdenumoxide and gadolinium oxide blended at such a rate as to meet thestoichiometric value for forming gadolinium molybdate is heated andmelted, a seed crystal is immersed in said melt and a single crystal ofgadolinium molybdate is grown according to the pull method, and theresulting single crystal is cooled; the improvement which comprises thecooling of said crystal during the cooling step in the temperature zoneof at least 900C. to 700C. at a cooling rate of over 200C./hr.

2. The method according to claim 1, wherein said cooling is conductedunder an oxygen-containing atmosphere.

3. A method according to claim 1, wherein the cooling in the temperaturezone of at least 900C. to 700C. is performed at a cooling rate of 200C.to l,500C./hr.

4. The method according to claim 1, wherein the cooling rate is about450C per hour or higher.

1. IN A METHOD FOR PRODUCING GADOLINIUM MOLYBDATE SINGLE CRYSTALS WHEREIN A POLYCRYSTAL OF GADOLINIUM MOLYBDATE OR A MIXTURE OF MOLYBDENUM OXIDE AND GADOLINIUM OXIDE BLENDED AT SUCH A RATE AS TO MEET THE STOICHIOMETRIC VALUUE FOR FORMING GADOLINIUM MOLYBDATE IS HEATED AND MELTED, A SEED CRYSTAL IS IMMERSED IN SAID MELT AND SINGLE CRYSTAL OF GADLOINIUM MOLYBDATE IS GROWN ACCORDING TO THE PULL METHOD, AND THE RESULTING SINGLE COOLED: THE IMPROVEMENT WHICH COMPRISES THE COOLING OF SAID CRYSTAL DURING THE COOLING STEP IN THE TEMPERATURE ZONE OF AT LEAST 900*C. TO 700*C . AT A COOLING RATE OF OVER 200*C/HR.
 2. The method according to claim 1, wherein said cooling is conducted under an oxygen-containing atmosphere.
 3. A method according to claim 1, wherein the cooling in the temperature zone of at least 900*C. to 700*C. is performed at a cooling rate of 200*C. to 1,500*C./hr.
 4. The method according to claim 1, wherein the cooling rate is about 450*C per hour or higher. 